The Kinetic Isotope Effect as a Probe of Spin Crossover in the CH Activation of Methane by the FeO+ Cation

Two‐state reactivity (TSR) is often used to explain the reaction of transition‐metal–oxo reagents in the bare form or in the complex form. The evidence of the TSR model typically comes from quantum‐mechanical calculations for energy profiles with a spin crossover in the rate‐limiting step. To prove the TSR concept, kinetic profiles for C? H activation by the FeO⁺ cation were explored. A direct dynamics approach was used to generate potential energy surfaces of the sextet and quartet H‐transfers and rate constants and kinetic isotope effects (KIEs) were calculated using variational transition‐state theory including multidimensional tunneling. The minimum energy crossing point with very large spin–orbit coupling matrix element was very close to the intrinsic reaction paths of both sextet and quartet H‐transfers. Excellent agreement with experiments were obtained when the sextet reactant and quartet transition state were used with a spin crossover, which strongly support the TSR model.     

The Kinetic Isotope Effect as a Probe of Spin Crossover in the C?H Activation of Methane by the FeO+ Cation

Two‐state reactivity (TSR) is often used to explain the reaction of transition‐metal–oxo reagents in the bare form or in the complex form. The evidence of the TSR model typically comes from quantum‐mechanical calculations for energy profiles with a spin crossover in the rate‐limiting step. To prove the TSR concept, kinetic profiles for C H activation by the FeO⁺ cation were explored. A direct dynamics approach was used to generate potential energy surfaces of the sextet and quartet H‐transfers and rate constants and kinetic isotope effects (KIEs) were calculated using variational transition‐state theory including multidimensional tunneling. The minimum energy crossing point with very large spin–orbit coupling matrix element was very close to the intrinsic reaction paths of both sextet and quartet H‐transfers. Excellent agreement with experiments were obtained when the sextet reactant and quartet transition state were used with a spin crossover, which strongly support the TSR model.     

Theoretical study of low-spin, high-spin, and intermediate-spin states of [Fe(III)(pap)2](+) (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato). Mechanism of light-induced excited spin state trapping

The mechanism of light-induced excited spin state trapping (LIESST) of [FeIII(pap)2]+ (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato) was discussed on the basis of potential energy surfaces (PESs) of several important spin states, where the PESs were evaluated with the DFT(B3LYP) method. The PES of the quartet spin state crosses those of the doublet and sextet spin states around its minimum. This means that the spin transition occurs from the quartet spin state to either the doublet spin state or the sextet spin state around the PES minimum of the quartet spin state. The PES minimum of the sextet spin state is slightly less stable than that of the doublet spin state by 0.18 eV (4.2 kcal/mol). This small energy difference is favorable for the LIESST. The doublet-sextet spin crossover point is 0.41 eV (9.6 kcal/mol) above the PES minimum of the sextet spin state. Because of this considerably large activation barrier, the thermal spin transition and the tunneling process do not occur easily. In the doublet spin state, the ligand to ligand charge transfer (LLCT) transition is calculated to be 2.16 eV with the TD-DFT(B3LYP) method, in which the pi orbital of the phenoxy moiety and the pi* orbital of the imine moiety in the pap ligand participate. This transition energy is moderately smaller than the visible light of 550 nm used experimentally. In the sextet spin state, the ligand to metal charge transfer (LMCT) transition is calculated to be at 2.36 eV, which is moderately higher than the visible light (550 nm). These results indicate that the irradiation of the visible light induces the LIESST to generate the sextet spin state but the reverse-LIESST is also somewhat induced by the visible light, indicating that the complete spin conversion from the doublet spin state to the sextet one does not occur, as reported experimentally.     

The Dynamics of the Reaction of FeO+ and H2: A Model for Inorganic Oxidation

Extensive density functional theory (DFT) calculations using the B3LYP functional were used to explore the sextet and quartet energy potential energy surfaces (PESs) of the title reaction, and as a basis to fit global analytical reactive PESs. Surface-hopping dynamics on these PESs reproduce the experimentally observed reactivity and confirm that hydrogen activation rather than spin-state change is rate-limiting at low reaction energy, where the main products are Fe⁺ and H₂O. A change in spin state is inefficient in the product region so that excited-state ⁴Fe⁺ is the dominant product. At higher energies, spin-allowed hydrogen atom abstraction to form FeOH⁺ predominates. At intermediate energy, a previously unexpected rebound mechanism contributes significantly to the reactivity.     

Collision-induced and infrared multiphoton dissociation studies on M(acetone)2 + (M=Al,Fe, Co,Cu, ScO) in the gas phase

In this paper we report an extension of our earlier study on the structure of Alfacetone)₂ ⁺ Collision-induced dissociation (CID) on MfacetoneXacetone-d₆)⁺ for M = Al, Fe, Co, and Cu yields primarily, if not exclusively, nearly equal amounts of acetone and acetone-d₆. Likewise, infrared multiphoton dissociation (IRMPD) at 10.6 μm yields, exclusively, nearly equal losses of the labeled and unlabeled acetones. These results suggest that the two acetone ligands bind in an equivalent fashion. Sc⁺ was also studied, which proved to be the most interesting. Sc⁺ reacts with acetone to form primarily ScO⁺, which undergoes higher order reactions leading to several products including ScO(acetone)₂ ⁺. IRMPD on this ion produces ScO(acetone-d₆)(CD₂CO)⁺, while its perdeuterated analog also produces ScO(acetone-d₆)⁺ in addition to ScO(acetone-d₆(CD₂CO)⁺. The IRMPD results are supplemented by studying the primary and higher order reactions of Sc⁺ with acetone, as well as the CID of ScO(acetone)₂ ⁺. Finally, a qualitative assessment of the infrared photodissociation cross sections is given. It is found that the relative photodissociation cross sections follow the orders Co(acetone-d₆)₂ ⁺ > Co(acetone)(acetone-d₆) > Co(acetone)₂ ⁺ and Co(acetone-d₆)⁺ > Co(acetone)⁺.     

Ab initio study on the reaction of Sc+ + CH4 → Sc+(SINGLE BOND)CH2 + H2

An ab initio study on the reaction of the ground state (³D) and the excited state (¹D) of Sc⁺ with methane was performed. Reaction channels on the singlet and triplet potential surface (PES) and the reaction mechanism are examined and discussed. Three regions of the potential surface was studied: the molecular complex, the C(SINGLE BOND)H insertion products, and the transition states for the reaction. Comparisons between singlet and triplet PESs show that the excited state (¹D) of Sc⁺ has more reactivity with methane than does the ground state (³D) due to the spin quantum number conservation with the more stable insertion intermediate. © 1997 John Wiley & Sons, Inc.     

Extreme Differences in the Interactions of `Bare' Fe+ and Fe2+ with Hydrogen Peroxide: Fenton Chemistry in the Gas Phase

The interaction of bare iron mono‐ and dications with hydrogen peroxide in the gas phase is studied by ab initio calculations employing the B3LYP/6‐311+G* level of theory. For the monocation, the quartet and sextet coordination complexes Fe(H₂O₂) are high‐energy isomers that easily interconvert to the more stable iron dihydroxide monocation Fe(OH) and hydrated iron oxide (H₂O)FeO⁺ (quartet) or dissociate into FeOH⁺+OH^. (sextet). On the dication surface, however, the order of stabilities is reversed in that Fe(H₂O₂)²⁺ (quintet) corresponds to the most stable doubly charged species, while the formal Fe^IV compounds Fe(OH) and (H₂O)FeO²⁺ are higher in energy.     

Structures,Unimolecular Fragmentations,and Reactivities of the Self‐Assembled Multimetallic/Peptide Complexes [Mnn(GlyGly‐H)2n−1]+ and [Mnn+1(GlyGly‐H)2n]2+

Complexes of Mn²⁺ with deprotonated GlyGly are investigated by sustained off‐resonance irradiation collision‐induced dissociation (SORI‐CID), infrared multiple‐photon dissociation spectroscopy, ion–molecule reactions, and computational methods. Singly [Mn_n(GlyGly‐H)_2n?1]⁺ and doubly [Mn_n+1(GlyGly‐H)_2n]²⁺ charged clusters are formed from aqueous solutions of MnCl₂ and GlyGly by electrospray ionization. The most intense ion produced was the singly charged [M₂(GlyGly‐H)₃]⁺ cluster. Singly charged clusters show extensive fragmentations of small neutral molecules such as water and carbon dioxide as well as dissociation pathways related to the loss of NH₂CHCO and GlyGly. For the doubly charged clusters, however, loss of GlyGly is observed as the main dissociation pathway. Structure elucidation of [Mn₃(GlyGly‐H)₄]²⁺ clusters has also been done by IRMPD spectroscopy as well as DFT calculations. It is shown that the lowest energy structure of the [Mn₃(GlyGly‐H)₄]²⁺ cluster is deprotonated at all carboxylic acid groups and metal ions are coordinated with carbonyl oxygen atoms, and that all amine nitrogen atoms are hydrogen bonded to the amide hydrogen. A comparison of the calculated high‐spin (sextet) and low‐spin (quartet) state structures of [Mn₃(GlyGly‐H)₄]²⁺ is provided. IRMPD spectroscopic results are in agreement with the lowest energy high‐spin structure computed. Also, the gas‐phase reactivity of these complexes towards neutral CO and water was investigated. The parent complexes did not add any water or CO, presumably due to saturation at the metal cation. However, once some of the ligand was removed via CO₂ laser IRMPD, water was seen to add to the complex. These results are consistent with high‐spin Mn²⁺ complexes.     

Theoretical study of the dehydrogenate reaction of H2S by Sc+(1D)

The reaction Sc⁺(¹D)+H₂S→Sc⁺S+H₂ is theoretically investigated by ab initio MO methods. Two possible reaction channels on the singlet potential surface (PES) and the reaction mechanism are examined and discussed. Three regions of the potential surface were studied, the molecular complex, the S‐H insertion products and the transition states for the reaction. In addition the singlet and triplet PESs of this reaction system are compared in an investigation the chemistry of excited electronic state. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 82: 60–64, 2001     

Octacyanidorhenate(V) Ion as an Efficient Linker for Hysteretic Two‐Step Iron(II) Spin Crossover Switchable by Temperature,Light, and Pressure

A two‐step hysteretic Fe^II spin crossover (SCO) effect was achieved in programmed layered Cs{[Fe(3‐CNpy)₂][Re(CN)₈]}?H₂O ( 1 ) (3‐CNpy=3‐cyanopyridine) assembly consisting of cyanido‐bridged Fe^II‐Re^V square grid sheets bonded by Cs⁺ ions. The presence of two non‐equivalent Fe^II sites and the conjunction of 2D bimetallic coordination network with non‐covalent interlayer interactions involving Cs⁺, [Re^V(CN)₈]^3? ions, and 3‐CNpy ligands, leads to the occurrence of two steps of thermal SCO with strong cooperativity giving a double thermal hysteresis loop. The resulting spin‐transition phenomenon could be tuned by an external pressure giving the room‐temperature range of SCO, as well as by visible‐light irradiation, inducing an efficient recovery of the high‐spin Fe^II state at low temperatures. We prove that octacyanidorhenate(V) ion is an outstanding metalloligand for induction of a cooperative multistep, multiswitchable Fe^II SCO effect.     

Spin‐State‐Controlled Photodissociation of Iron(III) Azide to an Iron(V) Nitride Complex

The generation of iron(V) nitride complexes, which are targets of biomimetic chemistry, is reported. Temperature‐dependent ion spectroscopy shows that this reaction is governed by the spin‐state population of their iron(III) azide precursors and can be tuned by temperature. The complex [(MePy₂TACN)Fe(N₃)]²⁺ (MePy₂TACN=N ‐methyl‐N ,N ‐bis(2‐picolyl)‐1,4,7‐triazacyclononane) exists as a mixture of sextet and doublet spin states at 300 K, whereas only the doublet state is populated at 3 K. Photofragmentation of the sextet state complex leads to the reduction of the iron center. The doublet state complex photodissociates to the desired iron(V) nitride complex. To generalize these findings, we show results for complexes with cyclam‐based ligands.     

Theoretical study of the reaction of Cr+ with SCO in gas phase

To elucidate the mechanism of reaction M⁺ + SCO, the reaction of Cr⁺ + SCO has been investigated using density functional theory (DFT) with the popular hybrid functional, B3LYP, in conjunction with 6‐311+G* basis set on both the sextet and quartet potential energy surfaces (PESs). To obtain an accurate evaluation of the activation barrier and reaction energy, the coupled cluster single‐point calculations using the B3LYP structures is performed. The crossing points (CPs) of the different PESs have been localized with the approach suggested by Yoshizawa and colleagues. The involving potential energy curve‐crossing dramatically affects reaction mechanism. The present results show that the reaction mechanism is insertion‐elimination mechanism both along the C? S and C? O bond activation branches, but the C? S bond activation is much more favorable than the C? O bond activation in energy. All theoretical results not only support the existing conclusions inferred from early experiment study, but also complement the pathway and mechanism for this reaction. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Automated reaction path searches for spin‐forbidden reactions

Many catalytic and biomolecular reactions containing transition metals involve changes in the electronic spin state. These processes are referred to as “spin‐forbidden” reactions within nonrelativistic quantum mechanics framework. To understand detailed reaction mechanisms of spin‐forbidden reactions, one must characterize reaction pathways on potential energy surfaces with different spin states and then identify crossing points. Here we propose a practical computational scheme, where only the lowest mixed‐spin eigenstate obtained from the diagonalization of the spin‐coupled Hamiltonian matrix is used in reaction path search calculations. We applied this method to the ^6,4FeO⁺ + H₂ → ^6,4Fe⁺ + H₂O, ^6,4FeO⁺ + CH₄ → ^6,4Fe⁺ + CH₃OH, and ⁷Mn⁺ + OCS → ⁵MnS⁺ + CO reactions, for which crossings between the different spin states are known to play essential roles in the overall reaction kinetics. © 2018 Wiley Periodicals, Inc.     

Methane Activation by Diatomic Molybdenum Carbide Cations

Metal carbide species have been proposed as a new type of chemical entity to activate methane in both gas‐phase and condensed‐phase studies. Herein, methane activation by the diatomic cation MoC⁺ is presented. MoC⁺ ions have been prepared and mass‐selected by a quadrupole mass filter and then allowed to interact with methane in a hexapole reaction cell. The reactant and product ions have been detected by a reflectron time‐of‐flight mass spectrometer. Bare metal Mo⁺ and MoC₂H₂⁺ ions have been observed as products, suggesting the occurrence of ethylene elimination and dehydrogenation reactions. The branching ratio of the C₂H₄ elimination channel is much larger than that of the dehydrogenation channel. Density functional theory calculations have been performed to explore in detail the mechanism of the reaction of MoC⁺ with CH₄. The computed results indicate that the ethylene elimination process involves the occurrence of spin conversions in the C?C coupling (doublet→quartet) and hydrogen atom transfer (quartet→sextet) steps. The carbon atom in MoC⁺ plays a key role in methane activation because it becomes sp³ hybridized in the initial stages of the ethylene elimination reaction, which leads to much lower energy barriers and more stable intermediates. This study provides insights into the C?H bond activation and C?C coupling involved in methane transformation over molybdenum carbide‐based catalysts.     

Theoretical study of ScCO2+

The structure, binding energy, and vibrational frequencies have been determined for ScCO₂⁺. The inserted OSc⁺CO structure in the ¹A′ state is the most stable isomer and lies 43.2 kcal/mol below the ground-state Sc⁺+ CO₂ asymptote. The linear η¹-O Sc⁺(SINGLE BOND)OCO ³Δ state is bound by a charge-quadrupole interaction and has a binding energy of 13.9 kcal/mol. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 523–528, 1997     

A CAS study on S‐loss and O‐loss dissociation mechanisms of the SO2+ ion in the C,D, and E states

In the present work, we mainly study dissociation of the C ²B₁, D²A₁, and E²B₂ states of the SO₂⁺ ion using the complete active‐space self‐consistent field (CASSCF) and multiconfiguration second‐order perturbation theory (CASPT2) methods. We first performed CASPT2 potential energy curve (PEC) calculations for S‐ and O‐loss dissociation from the X, A, B, C, D, and E primarily ionization states and many quartet states. For studying S‐loss predissociation of the C, D, and E states by the quartet states to the first, second, and third S‐loss dissociation limits, the CASSCF minimum energy crossing point (MECP) calculations for the doublet/quartet state pairs were performed, and then the CASPT2 energies and CASSCF spin‐orbit couplings were calculated at the MECPs. Our calculations predict eight S‐loss predissociation processes (via MECPs and transition states) for the C, D, and E states and the energetics for these processes are reported. This study indicates that the C and D states can adiabatically dissociate to the first O‐loss dissociation limit. Our calculations (PEC and MECP) predict a predissociation process for the E state to the first O‐loss limit. Our calculations also predict that the E²B₂ state could dissociate to the first S‐ and O‐loss limits via the A²B₂ ← E²B₂ transition. On the basis of the 13 predicted processes, we discussed the S‐ and O‐loss dissociation mechanisms of the C, D, and E states proposed in the previous experimental studies. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Substitution effects on olefin epoxidation catalyzed by Oxoiron(IV) porphyrin π-cation radical complexes: A dft study

The effects of peripheral fluorine atoms on epoxidation reactions of ethylene by oxoiron(IV) porphyrin cation radical complex in the quartet and sextet spin multiplicities are systematically investigated using the DFT method. The overall reaction routes are determined using a model system of ethylene and Fe(IV)OCl-porphyrin with substituted fluorine atoms. By obtaining the energy diagrams and electron- and spin-density difference contour maps of the transition states and intermediate compounds, we confirm that the electron-withdrawing by peripheral fluorine atoms enhances the reactivity as the number of fluorine atoms increases, as is observed experimentally. The intersystem crossing between the quartet and sextet spin multiplicities is discussed by means of the intrinsic reaction coordinate method. We conclude that the rate-determining step is located at the first transition state (TS1) for the activation of CC and FeO bonds, and the ground electronic state changes from quartet to sextet around the TS1. © 2019 Wiley Periodicals, Inc.     

Charge-remote and charge-proximate fragmentation processes in alkali-cationized fatty acid esters upon high-energy collisional activation. A new mechanistic proposal

The effect of the metal ion on the high-energy collision-induced dissociation (CID) of alkali metal-cationized n-butyl and methyl ester derivatives of palmitic and oleic acid was examined. The results show that the alkali metal ion has a pronounced effect and does not act as a mere ‘spectator’ ion with respect to the fragmentation process. While C–H cleavage is a dominant process for [M+Li]⁺ as well as [M+Na]⁺ precursor ions, C–C cleavage is also significant for the [M+Na]⁺ ions. Homolytic mechanisms involving the formation of a transient biradical cation are proposed which enable us to rationalize in a straightforward manner all product ions formed by both charge-remote and charge-proximate fragmentations. The mechanistic proposal is discussed in view of available knowledge on electron impact, CID and related processes. In order to predict how the alkali metal ion could affect the reactivity of the postulated biradical state formed following electronic excitation of the alkali metal-cationized molecules, quantum chemical calculations were performed on methyl and n-butyl acetate as model substances. The decreased spin density at the carbonyl oxygen atom in the biradical state may provide an explanation for the greater tendency towards C–C cleavage reactions of the sodium-cationized fatty acid esters relative to the corresponding lithium complexes. © 1988 John Wiley & Sons, Ltd.     

New insight into the electronic structure of iron(IV)‐oxo porphyrin compound I. A quantum chemical topological analysis

The electronic structure of iron‐oxo porphyrin π‐cation radical complex Por^·+Fe^IV?O (S? H) has been studied for doublet and quartet electronic states by means of two methods of the quantum chemical topology analysis: electron localization function (ELF) η(r) and electron density ρ(r). The formation of this complex leads to essential perturbation of the topological structure of the carbon–carbon bonds in porphyrin moiety. The double C?C bonds in the pyrrole anion subunits, represented by pair of bonding disynaptic basins V_i=1,2(C,C) in isolated porphyrin, are replaced by single attractor V(C,C)_i=1–20 after complexation with the Fe cation. The iron–nitrogen bonds are covalent dative bonds, N→Fe, described by the disynaptic bonding basins V(Fe,N)_i=1–4, where electron density is almost formed by the lone pairs of the N atoms. The nature of the iron–oxygen bond predicted by the ELF topological analysis, shows a main contribution of the electrostatic interaction, Fe^δ+···O^δ?, as long as no attractors between the C(Fe) and C(O) core basins were found, although there are common surfaces between the iron and oxygen basines and coupling between iron and oxygen lone pairs, that could be interpreted as a charge‐shift bond. The Fe? S bond, characterized by the disynaptic bonding basin V(Fe,S), is partially a dative bond with the lone pair donated from sulfur atom. The change of electronic state from the doublet (M = 2) to quartet (M = 4) leads to reorganization of spin polarization, which is observed only for the porphyrin skeleton (?0.43e to 0.50e) and S? H bond (?0.55e to 0.52e). © 2012 Wiley Periodicals, Inc.